1. Field of the Invention
The invention relates to a heater having an electrically conductive substrate and an image heating apparatus such as a heating and fixing apparatus carried on an image forming apparatus such as a copier or a printer.
2. Description of Related Art
In an image output apparatus such as a copier, a printer or a facsimile apparatus using a suitable image forming process such as electrophotographic, electrostatic recording or magnetic recording, a heating apparatus of the heat roller type has heretofore been used as a fixing apparatus for fixing a toner image formed and borne on a recording material by a transferring method or a direct method on the surface of the recording material.
The heating apparatus of the heat roller type basically comprises a metallic roller provided with a heater therein, and a pressure roller having elasticity and brought into pressure contact therewith, and a recording material as a member to be heated is introduced into a fixing nip part (pressure contact nip part) formed by the pair of rollers and is nipped and conveyed by the fixing nip part to thereby heat and pressurize a toner image and fix it on the recording material.
In such a heating apparatus of the heat roller type, however, the heat capacity of the roller is great and therefore, much time has been required to raise the surface of the roller to a fixing temperature. This also has led to the problem that to execute the image outputting operation quickly, the surface of the roller must be controlled to a certain degree of temperature even when the apparatus is not used.
So, heating apparatus devised to solve these problems include heating apparatuses of the film heating type disclosed, for example, in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, etc. filed by the applicant.
Such heating apparatus of the film heating type usually comprises a thin heat-resistant film, a heating member (hereinafter referred to as the heater) fixedly supported and disposed on one surface side of the film, and a pressure member disposed on the other surface side of the film in opposed relationship with the heater for bringing a member to be heated into close contact with the heater with the film interposed therebetween.
The member to be heated, i.e., in an image heating apparatus, a recording material having a toner image formed and borne thereon, is introduced and passed between the film and the pressure member in a pressure contact nip part formed by the pressure contact between the heater and the pressure member with the film interposed therebetween, whereby the visible image bearing surface of the recording material is heated by the heater through the film, and heat energy is imparted to the unfixed image, and the toner is softened and fused, whereby the heating and fixing of the image are done.
The film heating type is excellent for its quick starting property, and can greatly curtail the electric power consumption during the standby state, and can constitute a heating apparatus of the on-demand type.
Heretofore, in a fixing apparatus adopting such a film heating type, for example, as the heater, use has often been made of a ceramic substrate such as alumina as a substrate as in a ceramic heater, but this has posed the problems of the fragility or high cost of ceramics, and the unsuitability of ceramics for bending.
So, in Japanese Patent Application Laid-Open No. 9-244442 and Japanese Patent Application Laid-Open No. 10-275671, there is proposed a heating member (a conductive substrate heater, hereinafter referred to as the metallic heater) comprising a substrate given insulating property equal to that of a conventional ceramic substrate by forming an insulating layer on a metal material, and a heat generating resistance pattern, an electrically conducting pattern and an uppermost insulating sliding layer formed thereon.
By using a conductive substrate such as a metallic substrate high in heat conductivity as the substrate material of the heater, as described above, the temperature of the heater can be uniformalized over the entire area thereof and particularly, the temperature fall in the opposite end portions thereof can be prevented, whereby there can be formed a good image free of the unevenness of image such as uneven fixing, uneven luster or offset liable to occur over the length of a recording material. The temperature rise speed of the heater can also be improved, and it becomes possible to enhance the quick starting property. Further, as compared with ceramics or the like, the rupture strength itself of the metallic substrate is very high and therefore, there is no rupture of the substrate against heat stress or the like occurring during the sudden temperature rise of the heater, and the arising of a problem such as the breakage of the substrate in the manufacturing process can be suppressed, and it becomes possible to enhance productivity.
Also, to raise the temperature more quickly as the heater, it is necessary to efficiently give the heat of the heater to a member to be heated, and as a characteristic necessary for the heater, it is desirable to improve the heat conduction of the surface side of the heater (that surface side opposed to the member to be heated, or the paper passing surface side, hereinafter the same) and efficiently impart heat to the film and the recording material and at the same time, have on the back side of the heater (the surface side opposite to the surface side opposed to the member to be heated, or the non-paper passing surface side, hereinafter the same) such an adiabatic effect as will suppress the escape of heat to a heater supporting member or the like to the utmost.
The present invention has been made in view of the above-noted problems and an object thereof is to provide a heater which is high in strength and of which the adiabatic property of one surface is high and an image heating apparatus using the heater.
Another object of the present invention is to provide an image heating apparatus comprising a heater including a heat generating resistor, an electrically conductive substrate, an intermediate insulating layer formed between said resistor and said substrate and two surface insulating layers provided respectively on substantially entire areas of both surfaces of the two substrates, wherein the surface insulating layer on that surface side of the substrate which is opposed to a recording material is higher in heat conductivity than the surface insulating layer on the opposite surface side.
Still another object of the present invention is to provide a heater comprising:
a heat generating resistor;
an electrically conductive substrate; and
surface insulating layers provided on both surfaces of the substrate;
wherein the surface insulating layers differ in heat conductivity from each other.
Yet still another object of the present invention is to provide an image heating apparatus comprising a heater including a heat generating resistor, an electrically conductive substrate and surface insulating layers provided on both surfaces of the substrate, wherein the surface insulating layer on the side opposite to that surface of the substrate which is opposed to a recording material is greater in surface roughness than the surface insulating layer on the opposed surface side.
A further object of the present invention is to provide a heater comprising:
a heat generating resistor;
an electrically conductive substrate; and
surface insulating layers provided on both surfaces of the substrate;
wherein the surface insulating layers differ in surface roughness from each other.
Still a further object of the present invention is to provide an image heating apparatus comprising a heater including a heat generating resistor, an electrically conductive substrate, surface insulating layers provided on both surfaces of the substrate, and a temperature detecting element for detecting the temperature of the heater, wherein that area of the heater to which the temperature detecting element is opposed is higher in heat conductivity than the peripheral area thereof.
Yet still a further object of the present invention is to provide a heater comprising:
a heat generating resistor;
an electrically conductive substrate; and
surface insulating layers provided on both surfaces of the substrate,
wherein one surface of the heater has in a lengthwise portion thereof an area higher in heat conductivity than the peripheral area thereof.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.